*{
    user-select: none;
    /* 无法选中，整体感更强 */
}
body{
    background: url(../images/wallpaper.jpg) no-repeat;
    background-size: cover;
    background-attachment: fixed;
}
.teacher{
    position: absolute;
    top: 40%;
    margin-top: -200px;
    left: 40%;
    margin-left: -200px;
    /* absolute居中的一种方法 */
    background-color: whitesmoke;
    width: 700px;
    height: 600px;
    border-radius: 25px;
    text-align: center;
    padding: 5px 40px;
    box-sizing: border-box;
    /* 这样padding就不会影响大小 */
}
table
{
    border-collapse: collapse;
    margin: 0 auto;
    text-align: center;
}
table td, table th
{
    border: 1px solid #cad9ea;
    color: #666;
    height: 30px;
}
table thead th
{
    background-color: #CCE8EB;
    width: 100px;
}
table tr:nth-child(odd)
{
    background: #fff;
}
table tr:nth-child(even)
{
    background: #F5FAFA;
}
.s{
    width: 40px;
    height: 90px;
}